Digital assets are inherently fragile and are threatened by media
instability and format and hardware obsolescence. There are many
fundamental problems that can imperil digital information, for instance,
that practices undertaken to solve short-term problemscompression
or encryption, for instancemay result in an inability to "unscramble" information
in the long term; that digital works are often complex, and tracking
their interrelations and determining their boundaries over the long
term are likely to be difficult; that a lack of clarity about whose
responsibility it is to preserve digital material leaves it vulnerable
to falling through the cracks and becoming inaccessible to future
generations; and that translating digital information into new environments
often entails some change in meaning.6 This means that
it is vital to develop a preservation strategy at the very beginning
of the life cycle of a digital image collection if it is to be retained
as useful and valuable in the long term. Oya Y. Rieger has identified
four goals for digital preservation: (1) bit identity, ensuring
files are not corrupted and are secured from unauthorized use and
undocumented alteration; (2) technical context, maintaining interactions
with the wider digital environment; (3) provenance, maintaining a
record of the content's origin and history; and (4) references and
usability, ensuring users can easily locate, retrieve, and use the
digital image collection indefinitely. 7

The key to digital preservation is the establishment of a managed
environment. The default fate of analog objects is, arguably, to
survive (think of cuneiform tablets or papyrus scrolls), but without
persistent and regular intervention it is the fate of digital works
to perish. Digital preservation necessitates a paradigm shiftfrom
one where we subject objects to one-time or occasional conservation
treatments then leave them, perhaps for decades, in a temperature-
and humidity-controlled warehouseto a new approach where we, for
example, periodically review each work and copy it onto a new storage
medium, in all likelihood more than once per decade. Digital works
require ongoing management. All current digital preservation strategies
are flawed, or at best speculative, and thus a broad-based strategy
is the best current safeguard of any investment in digital imaging.
Over time it will be necessary to be vigilant as to both the condition
of the data and technological trends and to be prepared to reassess
policies accordingly. It will also be essential to have a long-term
commitment to staffing, continuous quality control, and hardware,
storage, and software upgrades.

The primary preservation strategy is to practice standards-driven
imaging. This means, first, creating digital image collections in
standard file formats at a high enough quality to be worth preserving,
and second, that sufficient documentation is captured to ensure that
the images will continue to be usable, meaning that all necessary
metadata is recorded in standard data structures and formats. One
complication here is that it is as yet unclear exactly what all the
necessary metadata for digital images is; some commentators are concerned
that too little metadata is captured, others that too much is. The
RLG preservation metadata elements are intended to capture the minimal
information needed to preserve a digital image. Various groups have
developed broader protocols or frameworks for digital preservation,
such as the OAIS model discussed below.

The secondary preservation strategy is redundant storage: images
and metadata should be copied as soon after they are created as is
practicable. Multiple copies of assets should be stored on different
media (most commonly, hard disks; magnetic tape, used for most automatic
backup procedures; and optical media such as CD-ROMs) and in separate
geographic locations; one of the most common causes of data loss
is fire or water damage to storage devices in local mishaps or disasters.
All media should be kept in secure archival conditions, with appropriate
humidity, light, and temperature controls, in order to prolong their
viable existence; additionally, all networked information should
be protected by security protocols.

Such redundancy is in accordance with the principle that "lots of
copies keep stuff safe," formalized by the LOCKSS system designed
at Stanford University Libraries to safeguard Web journals. Refreshing,
or the periodic duplication of files in the same format to combat
media decay, damage, or obsolescence, essentially extends this principle.
As yet, no robust preservation medium for digital data that is practical
for general use has emerged. Because of the availability of analog
media of proven longevity, some researchers suggest a hybrid approach
to preservation, in which digital material is derived from analog
material (such as photochemical intermediaries) or, alternatively,
analog backups or copies of digital material are created. (In this
context it is interesting to note the Rosetta Project, which aims
to provide a near-permanent archive of one thousand languages by
recording themas script readable with a powerful microscope rather
than binary codeon micro-etched nickel disks with a two-thousand-year
life expectancy.8)

Migration, the periodic updating of files by resaving them in new
formats so they can be read by new software, is where preservation
starts to become more problematic. Reformatting allows files to continue
to be read after their original format becomes defunct, but it involves
transforming or changing the original data, and continued transformation
risks introducing unacceptable information loss, corruption, and
possible loss of functionality. One suggested method of mitigating
this problem is technology preservation, which involves preserving
the complete technical environment necessary to access files in their
original format, including operating systems, original application
software, media drives, and so forth. However, it is unlikely that
this approach will allow the maintenance of viable systems over long
periods of time.

Emulation takes the alternative approach of using software to simulate
an original computer environment so that "old" files can be read
correctly, presuming that their bit streams have been preserved.
Emulation is a common practice in contemporary operating systemsfor
instance, code can be written to make programs designed for IBM-compatible
or Wintel personal computers run on Macintoshes as they would in
their native environment or to make programs designed for previous
versions of an operating system function in newer versions. Research
into emulation as a preservation strategy is ongoing. The initial
research direction sought to emulate the entire original hardware
and software environment and the functionalities it offered. More
recent approaches have suggested that it is more economically feasible
to use emulation to provide a viewing mechanism only, and that some
loss of functionality is acceptable.

Emulation holds out the seductive possibility of preserving the
original "look and feel" of archival data, but its large-scale practicality
remains to be demonstrated. However, it has already had some success
as a tool used in digital archaeologythe various methods
and processes undertaken to recover data from damaged or obsolete
media or hardware, defunct formats, or corrupted files when other
preservation strategies have failed or perhaps never been attempted.
Emulation was used to revive the BBC's Digital Domesday Book, an
extremely ambitious project stored on 1980s-era interactive videodiscs
that became inaccessible within fifteen years, in 2002. (The original
Domesday Book, a record of William the Conqueror's survey of England
compiled in 1086 by Norman monks, remains in good condition.) Emulation
has also been moderately successful in reviving obsolete arcade videogames,
re-creating solely through software the experience created by their
original hardware-specific environments.

Re-creation is a concept developed in the world of born-digital
multimedia or installation art. It postulates that if artists can
describe their work in a way that is independent of any platform
or medium, it will be possible to re-create it once its current medium
becomes extinct. Such a description would require the development
of a standard way of describing digital art analogous to musical
notation.

All or some combination of these strategies can be carried out in-house,
transferred to a third party such as a commercial data warehouse
service, or done in collaboration with other groups and institutions
through commercial or nonprofit resource-sharing initiatives. Resource
sharing may be the only practical way to conduct preservation for
many institutions in the long term. Examples of such initiatives
include the OCLC (Online Computer Library Center, Inc.) digital archival
service and the UK-based AHDS (Arts and Humanities Data Service)
data deposit service, both of which provide long-term management,
access, and preservation of digital assets. Transferring risk and
responsibility to a third party, however, does not by itself guarantee
preservationthe third party must be reliable and likely to continue
in existence. Trusted Digital Repositories: Attributes and Responsibilities,
a report written by RLG-OCLC in 2002, describes some of the characteristics
that would be required in such a storage facility.

The OAIS reference model can potentially provide a common conceptual
framework for the preservation and access of digital information,
and thus a common ground for discussion, collaboration, and research
in these areas. The model distills the entire life cycle of digital
objects, from ingest through storage and display, down to a fundamental
set of functions, relationships, and processes. It rests upon the
central concept of "information packages," meaning the data or bit
stream itself and the "representation information" that allows the
interpretation of the bit stream as meaningful information. These
may be regarded as analogous to the concepts of data and metadata.9

In reality, no one yet knows what the best preservation strategy
or combination of strategies will be. Whichever is chosen, it will
be necessary to run regularannual or biannualchecks on data integrity
and media stability and to be prepared to enter into a migration
program within five or so years. It is advisable to retain original
files over the long term if this is possible, but this will make
further demands upon management and storage capacity. Master files
should be afforded the maximum possible protection. Constant vigilance
and the consistent use of open standards and system-independent formats,
where possible, will be the best guarantee of the long-term viability
of a digital image collection.